The present invention relates to amplifiers, and more particularly to an amplifier employing digital switching of incremental signal sources for amplifying an amplitude and frequency varying input signal to high power levels.
In amplifier design, one important consideration is the efficiency at which the amplifier operates. Other important considerations include reliability, size, etc. It may be well appreciated that these considerations assume exaggerated importance in very high power amplifiers, such as those used in the modulating circuits of conventional broadcast transmitters. These audio amplifiers must amplify an input audio signal up to a power level ranging from several kilowatts to several tens of kilowatts.
An additional consideration in the selection of an amplifier design is the compatibility of that design with available solid state devices. Solid state devices, such as transistors, VFETs, SCR's, etc. are preferred over vacuum tube devices in view of their size, relative efficiency, and reliability. Unfortunately, such solid state devices as are readily available do not generally have sufficient power handling capabilities for use in standard very high power amplifier designs.
In the prior art, pulse duration modulation has been used effectively in increasing the efficiency of these high power amplifiers. Thus, as shown in the patent to Swanson, U.S. Pat. No. 3,506,920, high power amplification may be obtained by pulse width modulating a carrier signal in accordance with the changing level of an audio signal, amplifying the resulting bi-level pulse waveform, and then recovering an amplified audio signal by filtering the resulting amplified PDM signal. Even with these PDM amplifiers, however, no entirely satisfactory design has been found which employs high powered solid state devices in view of the existing limitations on power and speed of these devices.
One approach which has been taken in order to resolve this incompatibility is described in the patent to Swanson, U.S. Pat. No. 4,164,714, entitled "Polyphase PDM Amplifier". The amplifier described in this patent receives a time varying input signal and converts it into a plurality of modulated pulse trains of like polarity and frequency (each of which may, for example, be a PDM signal) and which are phase displaced from one another by a known amount. The pulse trains are combined to form a composite signal of increased magnitude and of substantially the same waveform as the input signal. Unlike previous PDM amplifiers, the design of this Polyphase PDM amplifier has been found to be compatible with existing solid state devices.